Heterogeneities in polymers constitute a potential serious threat to the quality of plastic products. Incompletely dispersed additives, impurities from polymerisation or compounding, spatial variation in molar mass and gel particles are a few examples of heterogeneities. For example for polyethylene and other polyolefins, gel particles are considered to be the major cause for problems with processing and the quality of end products. Gels can be classified into three basic categories:                crosslinked gels (unmeltable molecular structure)        rheological gels (meltable, but apparent because of significant differences in molecular structure)        phase separation gels (copolymer differences in molecular structure, i.e. domains of homopolymer in copolymer melt)        
The above-mentioned types are detectable by conventional methods.
Existence of crosslinked gel may be due to thermal oxidation and in such case its elemental composition will be different from the surrounding matrix polymer. This type is referred to as oxidised gel particles. A gel particle can also be formed in the polymerisation process and in this case it will have the same molecular composition as the matrix polymer, but a higher molar mass. This type is referred to as unoxidised gel particles. The detection of this last-mentioned type is difficult.
In order to speed-up quality control in the manufacturing process, gel particles should be detected already in the liquid (molten) state. Oxidised gel particles can be detected by optical microscopy because of their different material properties, and thus also refractive index or absorption properties different from the surrounding matrix phase.
Oxidation products absorb in the UV-range and also UV-microscopy can be used for their detection. On the other hand, unoxidised gel particles in a quiescent melt cannot be detected (visualised) by either classical optical or UV-microscopy, nor by spectroscopic methods, because they have the same elemental composition (refractive index) and the same repeating unit structure (spectroscopic properties) as the matrix polymer.